Anti-SARS-CoV-2 Seroprevalence in Northwestern Russia in 2020-2024

After the COVID-19 pandemic, the analysis of accumulated data could provide new insights to overcome further challenges for the healthcare system. The present study aimed to characterize the seropositivity levels in the context of COVID-19 morbidity in 2020–2022 and diagnostic, screening, and vaccination programs realization. For this purpose, retrospective analysis of anti-SARS-CoV-2 IgG levels in 41,295 serum samples harvested from the northwestern Russian population in 2020–2024 was performed. We revealed that seroprevalence gradually increased until the autumn of 2021, when vaccination became mandatory for certain groups and restrictions were placed on unvaccinated persons. In the following winter, the seroprevalence growth was accompanied by a massive Omicron spread that also impacted the herd immunity level. The proportion of seropositive subjects in the Northwestern Russia population reached the suggested herd immunity threshold, i.e., 80% seropositivity, only in 2022. Till the beginning of massive vaccination, the identified IgG levels in patients who reported suffering a non-COVID upper respiratory tract infection were significantly lower than in COVID-positive subjects. That suggests that the available diagnostic tests for SARS-CoV-2 provided patients and healthcare specialists with reliable information on the etiology of respiratory infections.